Multiple gap optically activated switch

ABSTRACT

Solid state optically activated switches in which a plurality of spaced  cacts are arranged in a line on one of the broad surfaces of a semiconductor wafer and one or more bridging electrodes are arranged on the opposite broad surface thereof. The two end contacts are the switch terminals. This arrangement directs the electric field and the load current into the bulk of the semiconductor in a zig-zag fashion to provide a high hold-off voltage and a short conduction path. Also the bridging electrode can be used as a control electrode. A novel circuit utilizing such switches is shown.

The invention described herein may be manufactured, used and licensed byor for the Government for governmental purposes without the payment tome of any royalties thereon.

This application is a division of application Ser. No. 596,966, filedApr. 5, 1984, now U.S. Pat. No. 4,633,286.

BACKGROUND OF THE INVENTION

In recent years the solid state optically activated switch has receivedincreasing attention because of its advantages in kilovolt pulsercircuits. This interest stems from several unique properties of theseswitches not found in prior art switches used in pulser circuits. Theseoptical switches comprise a bulk semiconductor usually of rectangularshape and can be intrinsic or doped. Since the semiconductor does notinvolve any p-n junctions, they can be scaled up in size to increasepower handling capacity. A pair of contacts are applied usually to thesame surface of the semiconductor wafer to define a gap between thecontacts. The gap length is chosen to provide a hold-off voltage higherthan the voltage to be switched, which is applied across the gap. Thishold-off voltage is a function of the semiconductor material, its dopantand whether or not the gap surface has been passivated (or oxidized).Application of light to the gap area which is sufficiently energetic tocause the formation of charge carriers in the form of electron and holepairs will rapidly close the switch and apply the high voltage to a loadin series therewith. If the light covers the entire gap area there is notransit time limitation and the switching action is extremely fast. Theswitching action is accomplished with low jitter, and has the capabilityof high pulse repetition frequencies. Another important feature of theseswitches is the electrical isolation of the light signal from theswitched power. One disadvantage of such an optical switch is the lowcurrent gain which results from the fact that approximately one photonof incident light is required to create one carrier pair. The presentinvention provides an optically activated switch of superior performancewith higher current gain than prior switches of this type.

SUMMARY OF THE INVENTION

The multiple gap optically actuated switch of the present inventioncomprises an even number of spaced contacts arranged in a line along oneof the broad faces of a bulk semiconductor wafer with a number ofbridging electrodes equal to one less than said even number arranged onthe opposite broad face of said wafer, with said bridging electrodesarranged to overlap pairs of said spaced contacts to form a plurality ofgaps within said wafer equal to one less than the total number of saidspaced contacts and said bridging electrodes. The high voltage to beswitched is applied to the two end contacts on said one face of saidwafer. Also, the novel switch can be alternately described as asemiconductor wafer having contacts on one broad surface thereof andbridging electrodes on the opposite broad surface arranged so that theelectric field before conduction caused by high voltage applied acrossthe end contacts on said one broad surface will produce internalelectric fields which zig-zag between the spaced contacts and saidbridging electrodes. This geometry directs the electric field into thebulk of the semiconductor chip where the hold-off voltage is higher thanthe surface hold-off voltage. Also with such a switch the surfacebetween the contacts will remain at high resistivity even duringconduction, thus preventing surface breakdown even during conductionthrough the bulk of the chip.

It is thus an object of this invention to provide an improved opticallyactivated solid state switch with a high hold-off voltage, high offresistance between the contacts thereof and a short and hence lowresistance conduction path, and which can be operated by a lowerintensity light source than prior art switches of this type.

Another object of the invention is to provide a multiple gap opticallyactivated switch in which the electric fields are directed into the bulksemiconductor material and in which the surface resistivity betweencontacts remains high both during voltage hold-off and duringconduction, and wherein a control electrode can be provided.

These and other objects and advantages of the invention will becomeapparent from the following detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art optically activated switch.

FIG. 2 is a two gap optically activated switch of the present invention.

FIG. 3 is a six gap switch of this type according to the presentinvention.

FIGS. 4 and 5 show applications of these novel switches in high voltagepulser circuits.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The prior art optically activated switch of FIG. 1 comprises arectangular block of either intrinsic or doped semiconductor material 7with two metal contacts 9 and 11 located on one of the broad facesthereof to define a gap between the contacts of length L. The contactsof this illustrative switch are square with sides of length W and thusthe gap area is L×W and if L is chosen as 1 mm and W as 2 mm, the gaparea will be 2 mm². The high voltage is applied to such a switch fromthe leads 15 and 13 connected to the contacts, as shown. The hold-offvoltage of such a switch is defined as the maximum voltage which can beapplied thereto without causing arcover and the resultant undesiredclosing of the switch. For prior switches of the type shown in FIG. 1,using a high resistivity semiconductor material in which the gap areahad been passivated using the best state of the art techniques, thehold-off voltage is approximately 20 kilovolts/cm. Thus for theillustrated device of FIG. 1 with a 1 mm wide gap, the maximum hold-offvoltage is approximately 2,000 volts. Further, in the device of FIG. 1the gap area which must be illuminated by the optical source used toinitiate conduction is, as stated, 2 mm². It will be apparent that witha switch such as that of FIG. 1, the electric field before conductioncaused by the applied high voltage will be localized along the surfaceof the semiconductor in the gap area. Also, during conduction most ofthe load current will be confined to this same area, which can causeoverheating.

It has been known in the prior art that breakdown voltages in theinterior of bulk semiconductor material are several times higher thanthe surface breakdown voltage. This so-called bulk breakdown voltage canexceed 100 kilovolts/cm. Optically activated switches have beensuggested to take advantage of this fact. Such switches have for examplecomprised a rectangular block of bulk semiconductor with the contacts ateither end thereof, however such switches have low switching efficiencysince high power light sources are required to produce the chargecarrier pairs throughout the volume of the semiconductor. Also, such aswitch provides a low resistivity surface path between the contacts whenin the on state. The present invention provides a switch of this typewhich takes advantage of the high bulk breakdown or hold-off voltage butdoes not have the disadvantages of these prior art switches.

The novel optically activated switch of FIG. 2 comprises a rectangularblock of bulk semiconductor material 17, of thickness T with a pair ofmetal contacts 19 and 21 on one of the broad surfaces thereof. Thesemetal contacts may be similar to those of the prior art switch of FIG.1, with a gap of 1 mm and square with 2 mm long sides. A third metalelectrode 23 is placed on the opposite broad surface of thesemiconductor block so that it bridges the two upper contacts. Thus if apositive high voltage is applied to contact 21 from lead 15, with thenegative high voltage terminal connected to the contact 19 via lead 13,as shown, two gaps within the semiconductor bulk material will beformed. The arrows 14 indicate the direction of the internal electricfield between contact 21 and bridging electrode 23 and the arrows 16 thefield between electrode 23 and contact 19. The switching light energy isapplied to these two areas of overlap of the contacts and the bridgingelectrode to operate the switch. The light can be applied to either oneside of the semiconductor block or the other, and the light wavelengthand the semiconductor material are chosen so that the light energy willpenetrate the entire semiconductor width which is covered by thecontacts.

Assuming that the thickness T of the semiconductor block 17 of FIG. 2 is0.4 mm, then the gap area to be illuminated by the switching lightsource would be this thickness multiplied by the length of each of thecontacts 19 and 21, doubled. This gap area is represented in FIG. 2 bythe area covered by the arrows 14 and 16. Thus the dual-gap switch ofFIG. 2 has a smaller gap area, namely 0.4 mm×2.0 mm×2 or 1.6 mm²,compared to the 2.0 mm² gap area of the prior art switch of FIG. 1. Thisresults in a reduction of the light required for switching action.Further, the gap length of the switch of FIG. 2 is twice the waferthickness of 0.8 mm in this example which would yield a hold-off voltageof approximately 0.8 mm×100 kilovolts/cm. which equals 8,000 volts whichis substantially more than the 2,000 volt figure for the prior artdevice of FIG. 1, which uses a semiconductor wafer of the samedimensions.

The embodiment of the invention in FIG. 3 includes six gaps formed byfour contacts 33, 35, 37 and 38 arranged in a line along one of thebroad surfaces of the semiconductor wafer 25, with three bridgingelectrodes 27, 29, and 31 on the opposite broad surface of the wafer.The electrode 27 bridges the contacts 33 and 35 and the electrode 29bridges contacts 35 and 37. Similarly electrode 31 bridges contacts 37and 38. With a positive voltage applied to contact 33 from lead 15 asshown and the negative high voltage terminal connected to contact 38 vialead 13, the internal electric field lines would be as indicated by thepairs of arrows 26. This six-gap switch provides a long gap length andhence a proportionally higher hold-off voltage.

FIGS. 4 and 5 show applications of these optically activated switches inhigh voltage pulse forming networks which are often used in radar sets.In FIG. 4 a high voltage source 39 has its positive terminal connectedto contact 21 of a first dual gap switch 42 like that of FIG. 2 andhaving the same reference numerals for the same parts. The other contact19 of this first dual gap switch is connected to an output terminal 46and one end of load 45. The other end of load 45 is grounded, as is thenegative side of source 39. The bridging electrode 23 of the firstswitch is connected to ground through a second similar dual gap switch48 comprising contacts 19 and 21. A light source 41 is arranged toilluminate the gaps of the first switch and light source 43 the gaps ofthe second switch 48. A control circuit 44 controls both of the lightsources. In operation, to generate an extremely short pulse such aspulse 52 at terminal 46, the control circuit first illuminates the twogaps of the first switch. This rapidly closes the first switch andproduces the leading edge of pulse 52. When it is desired to terminatethe pulse, the second switch 48 is operated by light source 43 inresponse to a control signal from circuit 44. This grounds the bridgingelectrode 23 of the first switch and rapidly terminates the pulse 52.Thus by controlling the timing of the outputs of the two light sources,a pulse of any desired duration can be generated.

FIG. 5 shows a dual gap switch 50 of the present invention in aconventional pulse forming circuit including a high voltage source 39,resistor 47, energy storing co-axial line 51, load resistor 53, andoutput terminal 57. With the switch 50 off, the voltage source chargesup the line 51 and when the switch 50 is activated by light source 49,the energy in line 51 is shorted to ground through the switch andthrough load resistor 53, thereby producing the output pulse 55.

While the invention has been described in connection with illustrativeembodiments, obvious variations therein will occur to those skilled inthe art, accordingly the invention should be limited only by the scopeof the appended claims.

I claim:
 1. A high voltage pulse forming network comprising:a pair ofsimilar optically activated switches; each switch comprising a bulksemiconductor wafer, a pair of spaced electrical contacts arranged in aline on one of the broad surfaces of said wafer, and a bridgingelectrode located on the opposite broad surface of said waferoverlapping and bridging the pair of spaced contacts; a source of highvoltage connected to an electrical contact of a first opticallyactivated switch; an output terminal connected to the other contact ofsaid first switch; the other optically activated switch having anelectrical contact connected to the bridging electrode of said firstswitch, with the other contact of the same connected to ground; firstlight source means for illuminating and activating said firstswitch;second light source means for illuminating and activating theother switch; and control means for sequentially enabling said first andsecond light source means, said second light source means being enableda predetermined short time after the first light source means is enabledto thereby disable the first optically activated switch.
 2. A pulseforming network as defined in 1 wherein the bulk semiconductor materialof the pair of switches and the light wavelength of said first andsecond light source means are selected so that the light energy willsubstantially penetrate the entire semiconductor width which is coveredby the contacts.